add accessibility documentation (#867)

docs/guide/accessibility.md

@@ -0,0 +1,64 @@
+Vue Select aims to follow the WAI-ARIA best practices for the 
+[combobox](https://www.w3.org/TR/wai-aria-practices-1.1/#combobox) and 
+[listbox](https://www.w3.org/TR/wai-aria-practices-1.1/#Listbox) widgets. 
+
+The UX of the component isdesigned around the HTML `<select>` element, while following the WAI-ARIA 
+specifications and best practices for creating accessible components. 
+
+## Combobox
+
+- [WAI-ARIA Combobox - Best Practices](https://www.w3.org/TR/wai-aria-practices-1.1/#combobox)
+- [WAI-ARIA Combobox - Specification](https://www.w3.org/TR/wai-aria-1.1/#combobox)
+
+## Listbox
+
+- [WAI-ARIA Listbox - Best Practices](https://www.w3.org/TR/wai-aria-practices-1.1/#Listbox)
+
+### Autocomplete
+
+WAI-ARIA suggests four forms of autocomplete for Comboboxes. Vue Select can be configured to provide
+these use cases.
+
+1. **No autocomplete** 
+
+    > When the popup is triggered, the suggested values it contains are the same regardless of the 
+    characters typed in the textbox. 
+  
+    ```html
+    <v-select :filterable="false" :options="['No Autocomplete', 'List Autocomplete']" />
+    ```
+    <v-select :filterable="false" :options="['No Autocomplete', 'List Autocomplete']" />
+
+2. **List autocomplete with manual selection**
+
+    > When the popup is triggered, it presents suggested values that complete or logically 
+    correspond to the characters typed in the textbox. The character string the user has 
+    typed will become the value of the textbox unless the user selects a value in the popup.
+
+    ```html
+    <v-select taggable :options="['No Autocomplete', 'List Autocomplete']" />
+    ```
+    <v-select taggable :options="['No Autocomplete', 'List Autocomplete']" />
+
+3. **List autocomplete with automatic selection**
+ 
+    > When the popup is triggered, it presents suggested values that complete or logically 
+    correspond to the characters typed in the textbox, and the first suggestion is automatically 
+    highlighted as selected. The automatically selected suggestion becomes the value of the textbox 
+    when the combobox loses focus unless the user chooses a different suggestion or changes the 
+    character string in the textbox.
+
+    ```html
+    <v-select :options="['No Autocomplete', 'List Autocomplete']" />
+    ```
+    <v-select :options="['No Autocomplete', 'List Autocomplete']" />
+
+4. **List with inline autocomplete** 
+    
+    > This is the same as list with automatic selection with one additional feature. The portion of 
+    the selected suggestion that has not been typed by the user, a completion string, appears inline
+     after the input cursor in the textbox. The inline completion string is visually highlighted and
+      has a selected state.
+      
+    🚧 Vue Select does not yet support this configuration, but it is on the roadmap 
+    [#865](https://github.com/sagalbot/vue-select/issues/865). 🚧